I served my mission in Portugal and have been back a few times.  Most recently went back in 2022 with my wife and had a great time.  I love the Algarve, but with only 3 days you would probably want to either stay in the Algarve or stay in Lisbon.  Here's a few ideas:

Sintra - Palácio da Pena, Castelo dos Mouros

Cabo de Roca - Westernmost point of continental Europe

Belem - Jerónimos Monastery, Torre de Belém, Padrão dos Descobrimentos

If you do decide to go to the Algarve I would suggest staying in Lagos or around there.  Send me a BM if you want more details on the Algarve.